Sensitive fluid balancing relief valve

ABSTRACT

A relief valve comprising a diaphragm for control of process pressure is provided. The diaphragm has a fluid pressure on a first side and a process pressure on a second side. The second side is engagable with a process void and at least one vent void such that when the process pressure is below the fluid pressure the diaphragm is engaged with the vent void. When the process pressure is above the fluid pressure the diaphragm is not engaged with the vent void.

FIELD OF THE INVENTION

[0001] The present invention relates to a new and improved modulatedpressure relief valve which balances vessel or line pressure against areference pressure signal.

BACKGROUND

[0002] There has been a long felt desire for a modulated pressure reliefvalve on a vessel for venting fluid when the pressure exceeds a desireddynamic set-point or threshold. A particular desire is for a modulatedpressure relief valve with a threshold that could vary according toprocess requirements and its upper limit could serve to avoid blowing asafety relief device such as an expendable rupture disc or pop-offsafety relief valve both of which are typically designed for infrequentuse. It is desirable to communicate the desired pressure set-point tothe relief valve via a reference pressure signal (typically air) that isequal to the desired vessel pressure.

[0003] Furthermore, it is desirable to have a modulating pressure reliefvalve which is very simple, with cleanable parts and with no narrowpassageways that can become plugged with debris or frozen product. Sucha device would be useful in many applications including the food,beverage, gelatin, and polymer industries, where the product can becomefrozen in critical passages, and where all crevices must be exposed tothe rinsing and cleaning processes.

[0004] Typical safety relief valves, such as those exemplified in U.S.Pat. No. 6,095,183, include a valve member biased by a spring intoengagement with a seat. If product pressure exceeds a predeterminedlevel, the force against the valve exceeds the biasing force of thespring causing the valve to lift from the seat to vent product pressure.

[0005] Typical diaphragm relief valves, such as those exemplified inU.S. Pat. No. 5,944,050, do present simple, cleanable surfaces yet theyclearly do not meet the dynamic set-point requirement desired in theart. Furthermore, the inaccuracies of the spring compression typicallyresult in wide variations in relieving pressures, often greater than10%, and the variations change over time.

[0006] Dynamically modulated relief valves are provided that use areference or pilot signal as exemplified in. U.S. Pat. No. 6,318,406.These typically involve complex spring and seal mechanisms. Besides theinherent robustness issues with the complex mechanisms, they are clearlynot acceptable for use in processes with debris, freezeable product, orrequiring cleanability.

[0007] Typical relief valves are quite insensitive to slight differencesin pressure, and typically have large cracking pressure biases. A highlysensitive relief valve would help maintain a constant vessel pressureduring changes in the upstream process environment which is a featurethat is very useful in many industrial steady-stateapplications,including constant-flow applications.

[0008] Further, there has been a long felt desire for a relief valvethat could balance vessel pressure exactly to a reference, or pilot,pressure. This would facilitate a complete vessel pressure controlsystem when used in conjunction with a simple instrument pressure senderand a, preferably no-loss, check valve.

[0009] Typically available pilot actuated relief valves cannot controlvessel pressure to the exact pressure of the reference signal.

SUMMARY OF THE INVENTION

[0010] It is an object of the invention to provide a pressure reliefdevice which is simple and cleanable, making it useful in a variety ofprocess applications where debris, freezeable product, or sanitaryrequirements prohibit recesses or crevices.

[0011] A further objective of the invention is the ability to controlvessel pressure via a remote pressure reference signal, such as commonlyused in industrial control systems.

[0012] Particular features of the relief device include, simplicity andcleanability, response to remote reference signal and high sensitivityto slight differences between the reference pressure and processpressure to achieve complete balance between vessel and referencepressure.

[0013] A further objective of the invention is to perfectly balancevessel pressure with the reference pressure, so as to facilitate acompletely automated vessel pressure control system, both filling andrelieving, by adding only a pressure sending device isolated from theprocess by a check valve.

[0014] In a preferred embodiment vessel pressure is generated in aforward direction by a pressure sender through a check valve while allreductions in vessel pressure are conducted by a modulating pressurerelief valve with reference pressure communicated directly from apressure sender.

[0015] Yet another objective of the invention is to provide a devicewith high pressure sensitivity, while preventing the unnecessary ventingof process fluids. This invention is imminently suitable for protectingsensitive instrumentation from moisture, foam or debris from the vesselventing process.

[0016] The present invention is a floating diaphragm relief device whichbalances vessel, or line pressure, against a reference pressure signal.A simple flexible diaphragm constrained on its periphery, separates thereference fluid chamber on one side from a specific seating surface onthe other side. This seating surface contains a connection to both thevessel, line or upstream process and vent or downstream environment. Theseating surface preferably contains one or more vent holes of varying,but defined, sizes, which, when the diaphragm is not fully seatedagainst them, can communicate from the vessel or line to the vent orlower pressure environment.

[0017] A particularly preferred embodiment is provided in a relief valvecomprising a diaphragm. The diaphragm has a fluid pressure on a firstside and a process pressure on a second side. The second side isengagable with a process void and at least one vent void such that whenthe process pressure is at or below the fluid pressure the diaphragm isengaged with the vent void. When the process pressure is above the fluidpressure the diaphragm is not engaged with the vent void.

[0018] Another preferred embodiment is provided in a relief valve. Therelief valve comprises a process housing which comprises a process voidand at least one vent void. The process void is in pressurecommunication with a process system, with a process pressure. The ventvoid selectively allows material to flow from the process system throughthe vent void. A reference housing is provided wherein the referencehousing contains a fluid at a predetermined pressure. A diaphragm isbetween the process housing and the reference housing and capable ofengaging with the vent void and the process void. When the referencepressure is equal or higher than the process pressure the diaphragm isengaged with the vent void. When the process pressure is higher than thereference pressure the diaphragm is not engaged with the vent void andthe process pressure can decrease through the vent void.

[0019] Another particularly preferred embodiment is provided in a reliefvalve. The relief valve comprises a diaphragm between a referencehousing and a process housing. The process housing comprises a referencevoid, in pressure communication with a process vessel at a processpressure, and a vent void. The reference housing comprises a fluid at areference pressure. When the reference pressure equals or exceeds theprocess pressure the diaphragm is engaged with the vent void and whenthe process pressure exceeds the reference pressure the diaphragm is notengaged with the vent void and the process pressure decreases by ventingthrough the vent void.

BRIEF DESCRIPTION OF DRAWINGS

[0020]FIG. 1 is an exploded perspective view of an embodiment of thepresent invention.

[0021]FIG. 2 is a cross-sectional view of an embodiment of the presentinvention.

[0022]FIG. 3 is a schematic representation of a system incorporating thepresent invention.

DETAILED DESCRIPTION

[0023] The invention is described with reference to the drawings. Thedrawings are provided to facilitate discussion of the invention and arenot intended to limit the invention in any way. In the drawings allsimilar elements are numbered accordingly.

[0024] An embodiment of the present invention is provided in FIG. 1. InFIG. 1, the modulated pressure relief valve, generally represented at 1,is shown in exploded perspective view. The modulated pressure reliefvalve, 1, comprises a diaphragm, 2, which is enclosed between anreference housing, 3, and a process housing, 4. The diaphragm, 2,comprises a reference surface, 20, and a process surface, 21, which isopposite the reference surface. For the purposes of the presentapplication the volume between the reference surface of the diaphragmand reference housing is referred to as the reference volume and thearea between the process surface of the diaphragm and the processhousing is referred to as the process volume. An optional seal, 5, canbe employed between the diaphragm and reference housing, between thediaphragm and process housing, or both to insure that the entireassembly is sealed relative to operating pressure ranges which areanticipated.

[0025] The reference housing, 3, comprising a reference coupling, 9,which engages with a reference pressure source, 30. The referencecoupling, 9, can be any coupling typically employed for connecting afluid source to a device. Threaded couplings, compression couplings,ferreted couplings, soldered couplings, welded couplings, barbedcouplings attached to flexible tubing and the like are imminentlysuitable for the demonstration of the invention. It would be well withinthe ability of one skilled in the art to attach a pressure system to acoupling and further description herein is not warranted.

[0026] The process housing, 4, comprises a process coupling, 10, whichattaches to a process system, 31, within which the pressure is to bemaintained. The process coupling, 10, can be any coupling typicallyemployed for connecting a pressure regulator, or pressure release to aprocess vessel or line. Threaded couplings, compression couplings,ferreled couplings, soldered couplings, welded couplings, barbedcouplings attached to flexible tubing and the like are imminentlysuitable for the demonstration of the invention. It would be well withinthe ability of one skilled in the art to attach a process system to acoupling and further description herein is not warranted.

[0027] The process housing, 4, comprises a process void, 11, which is inpressure communication with the process vessel such that the pressure atthe process void is proportional to the pressure in the process vessel.In a particularly preferred embodiment the pressure at the process voidis approximately equal to the pressure in the process vessel. In anotherembodiment the pressure at the process void is proportional to thepressure in the process vessel as would be realized when pressurereduction devices are utilized between the process vessel and modulatedpressure relief valve as known in the art. At least one vent void, 12,is provided in the process housing, 4. The vent void, 12, is in flowcommunication with the environment exterior to the process vessel. Aswill be more fully understood from further descriptions when thepressure exerted on the reference surface of the diaphragm exceeds thepressure on the process surface of the diaphragm at the vent processvent, 11, the process void and vent voids are engaged by the diaphragmthereby sealing the vent void and prohibiting material, or the pressurecreated by material, from passing through the vent void. When thepressure in the process void, 11, is sufficient to dislodge thediaphragm from sealing engagement with the process void the diaphragm ispersuaded towards the reference housing thereby disengaging thediaphragm from at least one vent void thereby forming a flow channelfrom the process void to the vent void whereby the pressure is releasedthrough the vent void.

[0028] A cross-sectional view of the modulated pressure relief valve isshown in FIG. 2 as it would appear when the pressure on the referencesurface, 20, of the diaphragm, 2, exceeds the pressure on the processsurface, 21, of the diaphragm. In this condition the reference volume,15, is maximized and the process volume is essentially non-existent. Asthe pressure exerted on the area of the process surface, 21, increasesdue to an increase in pressure in the process coupling, 10, thediaphragm is persuaded away from the process housing thereby distortingthe diaphragm. If the pressure on the process surface, 21, increases theprocess volume enlarges at the expense of the reference volume. As theprocess volume increases the diaphragm continues to be distorted untilthe diaphragm disengages with at least one vent void, 12, at which pointthe pressure is released through the vent void thereby allowing pressureto lower until the pressure is reduced to a pressure equal to thepressure in the reference volume at which point the process volumedecreases and the diaphragm reengages with the vent void. It would beapparent from the description that the higher the pressure differencebetween the vessel and the reference coupling the more distorted thediaphragm becomes and therefore the diaphragm disengages with more ventvoids.

[0029] An optional vent hood, 41, is provided whereby all materialtransiting through the vent void is captured and collected through avent port, 42. It would be readily apparent that each vent void may havea separate vent port attached thereto. The vent ports allow the ventedmaterial to captured and released as appropriate.

[0030] A clamping mechanism, 16, with a receiving inset, 17, is providedin FIG. 2 wherein the modulated pressure relief valve is received in theinset, 17, to maintain the reference housing and process housing insandwiched relationship with the diaphragm there between. The clampingmechanism could be a clamp, matching tabs, pliable ring seal or anymechanism capable of maintaining the modulated pressure relief valve insandwiched relationship. It is well within the ability of one skilled inthe art to utilize clamping mechanisms suitable for the applicationwhich would maintain the modulated pressure relief valve in theappropriate sandwiched relationship.

[0031] A schematic representation of a process system incorporating themodulated pressure relief valve is provided in FIG. 3. In FIG. 3, themodulated pressure relief valve, 1, is integral to a pressure controlsystem which includes a process vessel, 25, and pressure referencesource, 30. The process vessel, 25, is connected to the processcoupling, 10, of the modulated pressure relief valve, 1, by a processline, 31, as would be readily understood to one of ordinary skill in theart. The reference coupling, 9, of the modulated pressure relief valve,1, is connected to the reference pressure source, 30, by a process line,32, as would be readily understood to one of ordinary skill in the artto form a control loop. An optional, but preferred, feed back loop,comprising a check valve, 29, and optional separator, 31, providesoptimal control of the pressure within the process vessel. The checkvalve, 29, is preferably a one-way flow valve allowing flow towards thedirection of the process vessel but not counter thereto. The processvessel, 25, separator, 33, check valve, 29, and reference pressuresource, 30, are connected, preferably in series, by process lines, 28,27, and 26 respectively. The pressure in the process vessel ismaintained by venting through the modulated pressure relief valve, 1, orby replenishment through the check valve, 29. The system allows foraccurate, rapid pressure control in the process vessel. By way ofexample, if the operator desires to increase the pressure in the processvessel the pressure from the reference pressure source is increased. Thepressure above the diaphragm would then be increased and the air (orother fluid) would flow through the check valve. Once the pressure inthe process vessel equals the pressure supplied by the referencepressure source the check valve and modulated pressure relief valve areboth closed thereby the system pressure is neutral. It would be apparentthat, in typical manufacturing configurations, there is a possiblepressure drop in the lines leading to the vessel. This pressure drop maybe more than the corresponding pressure drop in the lines leading to themodulated pressure relief valve. This is advantageous for the presentinvention but not required. If the operator desires to lower thepressure in the process vessel the pressure supplied by the referencepressure source is decreased. The pressure exerted on the diaphragm willthen be lower on the reference face of the diaphragm and the processpressure will be allowed to vent until the pressure on each side of thediaphragm is rebalanced.

[0032] It is well known in the art that many control systems,particularly flow based control systems, do not adjust to a given levelbut instead typically go beyond the intended control level and thencorrect. This oscillatory approach is mitigated by the present inventionwhich represents a particular advantage. Due to the ability of themodulated pressure relief valve to rapidly vent a system, as illustratedin FIG. 3, balances the pressure introduction sufficiently to minimizethe build up of pressure above the desired pressure.

[0033] The knowledge provided herein and understanding related toselection and location of the diaphragm, in conjunction with theplacement, size, and shape of the vent voids, give the device acharacterized performance curve. In practice, the valve can modulatebetween the closed state and cracked state, wherein venting occurs, withpressure differentials below ¼ psi.

[0034] A diaphragm is situated across a surface containing both theprocess void, from vessel or line, and a variety of smaller vent voids.On the reference surface of the diaphragm, the pressure reference fluid,preferably air, provides balancing forces. The sensitivity of the deviceis provided by the size of the vent voids from which the diaphragm iseasily dislodged by low pressure differences. Greater relief volume isprovided by progressively larger vent voids. The distance between thediaphragm and the interior surface of the reference housing, whichdetermines the reference volume, is a critical parameter in predictingand controlling the sensitivity of the device.

[0035] The characteristics of the diaphragm are integral to theperformance of the device. Excellent sensitivity has been achieved witha flexible diaphragm with reinforcement fibers to prevent excessivestretching.

[0036] There are three fluid pressures acting on the diaphragm, processpressure, vent pressure, and reference pressure. The diaphragm serves tobalance the process pressure and reference pressure.

[0037] During normal balanced or modulating mode the diaphragm is drawninto a sealing relationship with the vent voids due to the pressuredifferential between the vent pressure and reference pressure. When thevessel pressure exceeds the reference pressure the area of the diaphragmbetween the vent voids is persuaded away from the outlet holes therebyallowing venting. This motion, however, is limited by the pressureexerted on the surface on the reference surface of the diaphragm. Thediaphragm is forced into an irregular pattern, with regions closelysurrounding the vent voids being unsupported by the interior surface ofthe process housing, and therefore trying to pull the diaphragm out ofcontact with the outlet hole. At the smallest holes, the ratio ofunsupported diaphragm area to outlet hole area can be very high, such as400:1 for example. When the ratio of pressure differential ratio, whichis defined as the differential Vessel—Reference divided byReference-Vent, rises to the area ratio, which is defined by theunsupported diaphragm area divided by the area of the vent void, thenthat particular vent void begins to crack open or modulate. The valvepreferably opens up the smaller vent voids first, and graduallyprogresses to the larger vent voids. At higher differential pressures,such as several psi, the entire hole pattern opens up to allow higherventing volumes.

[0038] The selection of hole size and spacing controls the relievingcapacity and the sensitivity of the device. The desired combination ofhigh relieving capacity and high sensitivity can be achieved bycombining both small holes and large holes in the same device. Thesmaller the size of the smallest vent voids, the greater the sensitivityof the device. In one embodiment a device with a minimum vent voiddiameter of 0.042″, would yield a cracking sensitivity of approximately¼ psi. Smaller vent voids would be expected to provide more sensitivity.In one embodiment the vent void is a frit with multiple flow pathsthrough a given vent void. The vent voids are preferably large enough toavoid pluggage by contaminants in the fluid stream, and small enough toprovide the required cracking sensitivity.

[0039] The number of small vent voids determines the relieving capacityof the device in the lower pressure differential range. While one verysmall vent void would provide a sensitive device that crack at very lowpressure differentials, it might not produce the needed effect on thesystem because the relieving volume of that hole might not be enough tocontrol the system pressure. Therefore, a number of small vent voids maybe provided such that the sum of their cross sectional area relates,proportionally, to the expected required relieving capacity. A vent voidhousing comprising 8 small holes, symmetrically arranged, is suitablefor demonstration of the present invention.

[0040] Larger vent voids contribute to higher relieving capacity at thehigher differential pressures. The ultimate relief capacity will beproportional to the sum total of all vent void cross sectional areas.Depending on the overall space available, a large hole size should beselected which provides enough capacity per hole for the processconditions without being so large as to have inadequate sensitivity. Thediameter of the larger holes determines the differential pressures atwhich the higher relief flows can occur. Large vent voids ofapproximately 0.15′ diameter, would be expected to begin cracking openin the 2-5 psi range.

[0041] The minimum and maximum vent void sizes are based on the desiredsensitivity of the valve in the low and high flow regimes and thecontaminant characteristics of the fluid. Intermediate vent void sizesmay be selected to assure good relieving capacity in the mediumdifferential pressure range. The number of vent voids should be selectedto achieve the desired maximum flow capacity. The full open flow of thevalve can be approximated by analyzing the parallel transmittanceprovided by the parallel outlet vent voids. Diaphragm size, typicallydefined by the diameter, is preferably selected to allow for areasonable spacing ratio between vent voids which is based, in part, ondiaphragm movement gap or the maximum distance between the referencesurface and interior of the reference housing.

[0042] The size of the process void is not a critical design parameter,but is preferably sized so that both the cross sectional area of theprocess void, and the cylindrical opening area between the process voidand the retracted diaphragm (periphery multiplied by diaphragm movementgap) are both significantly larger than the sum total cross sectionalarea of all the vent voids.

[0043] The diaphragm movement gap, or the maximum distance between thereference surface, 20, and the reference housing, is a criticalparameter. It affects the valve sensitivity and affects the optimumspacing of the vent void. The gap is preferably great enough that thecylindrical opening area (defined above), when the diaphragm is fullyretracted against the reference housing is larger than the sum total ofthe vent voids. A total diaphragm movement gap of 0.1 inch is suitablefor demonstration of the invention.

[0044] Each vent void must have an adequate free area around it so thatthe diaphragm can form a dimple shaped unsupported area. The size ofthis spacing is not related to the vent void size, but rather the gapdistance that the diaphragm can move (distance between seat surface toreference housing surface). Increased vent void spacing tends toincrease sensitivity up to a threshold ratio, and has less impact abovethat threshold ratio. This threshold ratio is dependant on diaphragmstiffness. A minimum hole spacing of 0.4 inches is adequate fordemonstration of the present invention. The diaphragm movement gap ofapproximately 0.1 inch results in a vent void spacing to movement gapratio of approximately 4:1.

[0045] The location of the vent voids is not a critical designparameter, assuming adequate spacing is provided. However, it isrecommended that the smaller vent voids be located closer to the processvoid.

[0046] Another variable in the performance of the valve is the locationof the diaphragm's peripheral constraint relative to the gap between theprocess surface and the reference surface. Offset of the diaphragmconstraint in the direction of the reference surface tends to bias thevalve to relieve at lower, or even negative differential pressures,which can result in a unnecessary flow or waste of fluid. Good balancedperformance, with negligible waste flow, can be achieved in the testeddesign by constraining the diaphragm directly along the process surface.The offset can be used to create a pressure bias. The modulated pressurerelief valve can be configured utilizing shaped surfaces, such asconcave or convex, on the diaphragm, process housing or both. Ribbing inthe diaphragm, or process housing, can also be effectively utilized toalter the pressure bias between the process pressure and referencepressure at which venting occurs. If the modulated pressure relief valveis configured with an offset, for example, wherein the diaphragm is awayfrom the process surface, for example, a positive pressure bias can becreated wherein the modulated pressure relief valve vents at a processpressure which is lower than the reference pressure. This is alsoreferred to in the art as an opening bias. Alternatively, the modulatedpressure relief valve can be configured such that a negative pressurebias is created wherein the process pressure must be higher than thereference pressure prior to venting. This would be referred to in theart as a closing bias.

[0047] The overall size of the diaphragm area ultimately determines themaximum possible relieving capacity. Increasing the overall size of thediaphragm area provides for greater potential relieving capacity, andcan, when carefully combined with vent void size, spacing, and movementgap selection, offer greater pressure sensitivity at a given relievingcapacity. A free diaphragm diameter of 2.9 inches is suitable fordemonstration of the present invention.

[0048] The diaphragm's physical characteristics affect the relievingperformance of the device. A reinforced elastomeric diaphragm, which ispliable to bending but is resistance to planar stretching, promoteshigher sensitivities than un-reinforced diaphragms by increasing theeffective size of the unsupported area around each vent void. Inpractice, a reinforced elastomeric diaphragm should be selected that isthin enough to be pliable, yet thick enough to withstand full systempressure stretched across the larger holes. Elastomeric stiffness, inDurometers, is not believed to be a critical factor for thin diaphragms.A {fraction (1/32)}″ thick fabric reinforced rubber sheet with a typicalor moderate stiffness, such as a Shore A durometer in 50-80 range, isparticularly suitable for demonstration of the present invention.

[0049] The reference pressure source is a fluid source wherein thepressure of the fluid source can be accurately controlled. Particularlypreferred fluids include gases. A particularly preferred gas comprisesnitrogen with a most preferred fluid being air. Liquids can also beemployed but are less desirable.

[0050] The separator is preferably a chamber which allows fluids toseparate from liquids in a stream. Separators are commonly employed toremove moisture from air streams wherein the air stream originates froma compressed air source. It is not uncommon for moisture to be entrainedwith the air flow and a separator allows the moisture to separate fromthe air flow. A separator can also provide a reservoir which acts toabsorb, or dampen, rapid changes in pressure without transmitting therapid pressure change further down the process line. For example, withwater flow systems, an air reservoir may be employed to eliminate thephenomenon commonly referred to as “hammering” or “bumping” due to rapidchanges in pressure.

[0051] The process system is any system commonly employed inmanufacturing environments wherein the pressure must be maintained at orbelow an upper maximum. The present invention is suitable for use withfixed reaction kettles where the pressure in the reaction kettle must bemonitored and with flow systems wherein the pressure build of flowingmaterials must be monitored.

[0052] It would be well within the ability of those with ordinary skillin the art to capture the material as it escapes form the vent voids orto redirect the material to a location of minimal concern.

[0053] The invention has been described with particular reference to thepreferred embodiments which are intended to enable one of ordinary skillin the art to demonstrate the present invention. The preferredembodiments are not intended to limit the scope of the present inventionwhich is set forth in the claims appended hereto.

1. A relief valve comprising a diaphragm wherein said diaphragm has afluid pressure on a first side and a process pressure on a second sideand said second side is engagable with a process void and at least onevent void such that when said process pressure is below said fluidpressure said diaphragm is engaged with said vent void and when saidprocess pressure is above said fluid pressure said diaphragm is notengaged with said vent void.
 2. The relief valve of claim 1 furthercomprising multiple vent voids.
 3. The relief valve of claim 2comprising a first vent void and a second vent void wherein said firstvent void is larger than said second vent void.
 4. The relief valve ofclaim 3 wherein said first vent void is closer to said process void thansaid second vent void.
 5. The relief valve of claim 2 wherein each saidvent void is the same distance from said process void.
 6. The reliefvalve of claim 1 further comprising a process housing wherein saidprocess housing comprises said process void and said vent void.
 7. Therelief valve of 4 further comprising a reference housing wherein saiddiaphragm is between said reference housing and said process housing. 8.The relief valve of claim 7 wherein when said process pressure is higherthan said reference pressure said diaphragm is persuaded towards saidreference housing.
 9. A process control system comprising: a processvessel; a reference pressure source; a control loop between said processvessel and said reference pressure source wherein said control loopcomprises a relief valve in accordance with claim 1; and a feed backloop between said process vessel and said reference pressure source. 10.A relief valve comprising: a process housing comprising a process voidand at least one vent void wherein said process void is in pressurecommunication with a process system, with a process pressure, and saidvent void selectively allows material to flow from said process systemthrough said vent void; a reference housing wherein said referencehousing contains a fluid at a predetermined pressure; and a diaphragmbetween said process housing and said reference housing capable ofengaging with said vent void and said process void wherein when saidreference pressure is higher than said process pressure said diaphragmis engaged with said vent void and when said process pressure is higherthan said reference pressure said diaphragm is not engaged with saidvent void and said process pressure can decrease through said vent void.11. The relief valve of claim 10 wherein said fluid is a gas.
 12. Therelief valve of claim 11 wherein said gas comprises nitrogen.
 13. Therelief valve of claim 10 further comprising multiple vent voids.
 14. Therelief valve of claim 13 comprising a first vent void and a second ventvoid wherein said first vent void is larger than said second vent void.15. The relief valve of claim 14 wherein said first vent void is closerto said process void than said second vent void.
 16. The relief valve ofclaim 13 wherein each said vent void is the same distance from saidprocess void.
 17. A process control system comprising: a process vessel;a reference pressure source; a control loop between said process vesseland said reference pressure source wherein said control loop comprises arelief valve in accordance with claim 10; and a feed back loop betweensaid process vessel and said reference pressure source.
 18. A reliefvalve comprising: a diaphragm between a reference housing and a processhousing wherein: said process housing comprises a reference void, inpressure communication with a process vessel at a process pressure, anda vent void; and said reference housing comprises a fluid at a referencepressure; wherein when said reference pressure exceeds said processpressure said diaphragm is engaged with said vent void and when saidprocess pressure exceeds said reference pressure said diaphragm is notengaged with said vent void and said process pressure decreases byventing through said vent void.
 19. The relief valve of claim 18 whereinsaid area between said reference housing and said diaphragm is areference volume and when said process pressure exceeds said referencepressure said reference volume decreases.
 20. The relief valve of claim18 wherein said fluid comprises nitrogen.
 21. The relief valve of claim18 comprising a first vent void and a second vent void wherein saidfirst vent void is between said process void and said second vent void.22. The relief valve of claim 21 wherein said first vent void is smallerthan said second vent void
 23. The relief valve of claim 18 wherein saidprocess pressure further comprises a pressure bias.
 24. A processcontrol system comprising: a process vessel; a reference pressuresource; a control loop between said process vessel and said referencepressure source wherein said control loop comprises a relief valve inaccordance with claim 18; and a feed back loop between said processvessel and said reference pressure source.
 25. The process controlsystem of claim 24 further comprising a check valve in said feed backloop between said process vessel and said reference pressure source. 26.The process control system of claim 25 further comprising a separator insaid feed back loop.